A diagram which shows the interconnection of process equipment and the instrumentation used to control the process. In the process industry, a standard set of symbols is used to prepare drawings of processes. The instrument symbols used in these drawings are generally based on International Society of Automation (ISA) Standard S5. 1.

The primary schematic drawing used for laying out a process control installation.

P&IDs play a significant role in the maintenance and modification of the process that it describes. It is critical to demonstrate the physical sequence of equipment and systems, as well as how these systems connect. During the design stage, the diagram also provides the basis for the development of system control schemes, allowing for further safety and operational investigations, such as a Hazard and operability study commonly pronounced as HAZOP.

Based on Standard ANSI/ISA S5.1 and ISO 14617-6, the P&ID is used for the identification of measurements within the process. The identifications consist of up to 5 letters. The first identification letter is for the measured value, the second is a modifier, 3rd indicates passive/readout function, 4th - active/output function, and the 5th is the function modifier.

Letter

Column 1 (Measured value)

Column 2 (Modifier)

Column 3 (Readout/passive function)

Column 4 (Output/active function)

Column 5 (Function modifier)

A

Analysis

Alarm

B

Burner, combustion

User choice

User choice

User choice

C

User's choice (usually conductivity)

Control

Close

D

User's choice (usually density)

Difference

Deviation

E

Voltage

Sensor

F

Flow rate

Ratio

G

User's choice (usually gaging/gauging)

Gas

Glass/gauge/viewing

H

Hand

High

I

Current

Indicate

J

Power

Scan

K

Time, time schedule

Time rate of change

Control station

L

Level

Light

Low

M

User's choice

Middle / intermediate

N

User's choice (usually torque)

User choice

User choice

User choice

O

User's choice

Orifice

Open

P

Pressure

Point/test connection

Q

Quantity

Totalize/integrate

Totalize/integrate

R

Radiation

Record

Run

S

Speed, frequency

Switch

Stop

T

Temperature

Transmit

U

Multivariable

Multifunction

Multifunction

V

Vibration, mechanical analysis

Valve or damper

W

Weight, force

Well or probe

X

User's choice (usually on-off valve as XV)

X-axis

Accessory devices, unclassified

Unclassified

Unclassified

Y

Event, state, presence

Y-axis

Auxiliary devices

Z

Position, dimension

Z-axis or Safety

Actuator, driver or unclassified final control element

For reference designation of any equipment in industrial systems the standard IEC 61346 (Industrial systems, installations and equipment and industrial products — Structuring principles and reference designations) can be applied. For the function Measurement the reference designator B is used, followed by the above listed letter for the measured variable.